1. Field of the Invention
The present invention relates to a method for the production of natural gas from methane hydrate deposits. More particularly, the present invention is directed to the release of methane from methane hydrates.
2. Description of the Related Art
Methane hydrate deposits are abundant throughout the world and have been estimated to represent by far the greater portion of the world's fossil energy reserve. Within the United States alone, methane hydrates represent an estimated 200,000 Trillion cubic feet (Tcf) of the total 227,500 Tcf of known natural gas reserves. The methane hydrate deposits, occurring at great depths primarily in the oceans, dwarf the total known combined oil and non-hydrate gas reserves. With the United States largely dependent upon imported fuels, there is an urgent need for a method to economically produce natural gas from the abundant United States methane hydrate reserves. Unfortunately, it has not yet been demonstrated that methane can be recovered from dissociated methane hydrates economically.
For in-situ dissociation, three approaches exist. One method involves heating the methane hydrate. This requires only about ten percent of the trapped gas heating value, assuming no heat losses. However, it has been found that pumping a heated fluid from the surface to the methane hydrate deposit results in such high heat loss from transporting the heated fluid downhole that essentially all of the heating value of the recovered methane is consumed to supply the needed energy expended in the recovery process. For deep deposits, all the heat is lost in transit downhole. In-situ combustion would minimize such transit heat losses but would be difficult to establish in a hydrate bed and would result in undesirably high bed temperatures.
A second method for in-situ dissociation involves reducing the in-situ pressure to a value below the methane hydrate dissociation pressure. However, the dissociation energy must still be supplied to the formation. Consequently, the methane hydrate formation temperature decreases thereby requiring even lower pressures for dissociation or heating of the deposit. Accordingly, this approach typically requires mining the solid methane hydrates and pumping a slurry to the surface. Such a mining system has yet to be demonstrated to be economically feasible.
Another method for in-situ dissociation involves pumping carbon dioxide downhole to displace methane from the methane hydrates by formation of carbon dioxide hydrates. However, this method has not been demonstrated as feasible as the reaction is slow at the deposit temperatures. In addition, conditions in a stable hydrate bed are appropriate for the formation of new methane hydrate from methane and water. Again, it is important in this method to raise the temperature of the deposit to minimize the reformation of methane hydrates.
It has now been found that heat losses incurred in providing a heated fluid for injection downhole into a methane hydrate bed can be substantially reduced by combusting a fuel downhole within the well casing and tempering the temperature of the combustion product gases by adding fluid in sufficient quantity to produce a heated fluid of a desired temperature for injection into the hydrate bed. Further, if the fuel comprises a compound containing carbon, carbon dioxide is produced. Thus, heat released by carbon dioxide hydrate formation is available to supply a portion of the heat required for methane hydrate decomposition.